The present invention relates to centrifugal pumps in general, and more particularly to improvements in impellers for use in centrifugal pumps, especially in centrifugal pumps which serve to effect forced circulation of contaminated fluids in the primary circuits of nuclear reactor plants. Still more particularly, the invention relates to improvements in centrifugal pump impellers of the type wherein the body of the impeller is surrounded by a wear ring which is shrunk onto and/or otherwise secured to the peripheral surface of the body.
It is already known to provide the impeller of a centrifugal pump with a wear ring which surrounds the periphery of and is normally shrunk onto the impeller body so that it must be destroyed in order to allow for separation from the body. It is also known to provide a wear ring with a flange or collar which overlies a portion of one end face of the impeller body and is separably secured to the latter. Still further it is known to employ in such impellers wear rings with cylindrical internal surfaces of constant diameter from end to end or with stepped cylindrical internal surfaces. When the need arises, a wear ring which has undergone a maximum permissible amount of wear must be detached from the impeller body and replaced with a fresh wear ring. The replacement involves treatment of the impeller in a suitable machine tool which removes the material of the spent or damaged wear ring preparatory to attachment of a fresh water ring. In some instances, the entire spent or damaged wear ring can be forcibly detached from the body of the impeller so that there is no need for removal of the material of the wear ring in a lathe or an analogous machine tool. As a rule, spare wear rings are held in storage in a condition such that a certain amount of their material must be removed by machining preparatory and/or subsequent to attachment to the body of an impeller. For example, a fresh water ring which is held in storage has an outer diameter exceeding the desired outer diameter of the composite impeller and an inner diameter which is smaller than required to allow for immediate application of such wear ring onto the body of an impeller.
When the pump which embodies an impeller of the above outlined character is used in the primary circuit of a nuclear reactor plant, the impeller is necessarily contaminated so that great care must be exercised during removal of a spent or damaged wear ring in order to confine the radiation and to avoid damage to the health of the workmen. In most instances, removal or replacement of a spent wear ring is preceded by a time-consuming, costly and complex decontamination which, in turn, is followed by removal of the material of the spent wear ring by resorting to a rotary material removing tool. Furthermore, a fresh wear ring must be applied to a radiation-emitting impeller body which involves additional substantial expenditures in connection with adequate shielding of attendants from radiation. Moreover, the replacement of a spent or damaged wear ring under the just outlined circumstances is cumbersome and contributes significantly to the maintenance cost of the entire reactor plant or any other installation in which the composite impeller is put to use.